1. Field of the Invention
The invention relates to a method of reconstructing an object image in a high-resolution electron microscope, in which an electron wave function is obtained at an exit plane of the object from at least one electron hologram recorded in a Fourier space, which hologram is composed of a central frequency domain CB=I.sub.hol,0 (G) and two sidebands SB+=I.sub.hol,+ (G) and SB-=I.sub.hol,- (G).
The invention also relates to a high-resolution electron microscope suitable for use of such a method.
2. Description of the Related Art
Three factors by which the information to be obtained from the object is influenced can be distinguished in an electron microscope. These factors are: the interactions of the electrons with the object, the transmission of the information-carrying electron beam in the microscope and the recording of the image.
An electron wave in high-resolution electron microscopy is, in a good approximation, a monochromatic, coherent beam and can be represented as a uniform plane wave incident on an object and subsequently propagating through the object. The electron wave is then phase and amplitude-modulated in conformity with the structure of the object as a result of the interactions between the electron beam and the object. In the case of an ideal scattering, the electron state before and after scattering can be accurately determined and information about the interaction and hence information about the object can be obtained from the change of state. For very thin specimens (having a thickness of the order of 1-3 nm) mainly the phase will be modulated so that a phase object is concerned in this case. However, for thicker specimens them is not necessarily a simple relationship between the projected structure of the object and the electron wave function at the exit plane of the object, because non-linear effects occur during the diffraction process due to multiple scattering of the electrons on the atoms present in the object. This leads to electron beams whose amplitude is non-linearly dependent on the structure of the object.
Subsequently the electron wave provided with image information is propagated towards the detector via the electron-optical system of the electron microscope so that a high magnification is realised at the expense of aberrations such as spherical aberration and focusing effects. Due to these aberrations the phase of the electron wave at the level of the back-focal plane of the objective lens of the electron microscope will be distorted in known manner. These phase distortions are represented by what is known as the phase transfer function of the electron microscope.
Finally, the electron intensity is recorded at the detector, which intensity represents the probability distribution of the electron wave in the image plane. However, during this recording operation the phase information is lost, which information is essential for describing the interference process in the electron microscope.
After the image information has been recorded on a detector, the electron wave function at the exit plane of the object comprising the actual phase and amplitude information of the object can be reconstructed by means of image processing methods, while the effect of the aberrations of the electron microscope on the image information is eliminated. As compared with the conventional point resolution of the electron microscope, the resolution can be considerably enhanced in this way. Point resolution is understood to mean the smallest detail that can be distinguished after formation of the image in the microscope at optimum, or Scherzer, focus, and yields directly interpretable information about the object, but only if an object can be described as a thin phase object. The point resolution is mainly determined by the spherical aberrations of the objective lens and the wavelength of the electron beam.
Such image reconstructions can be performed by using different methods. A first method is known from U.S. Pat. No. 5,134,288. This Patent describes an image reconstruction method with reference to focal variations in conventional high-resolution electron microscopy. In this method a series of images of one and the same object portion is recorded at varying defocus values. A Fourier transform and a linear combination is performed on this series of images so that the linear and non-linear image information are separated from each other. Subsequently, specific image information can be selected by optimizing the linear image formation. Linear image information is to be understood to mean the part of the image contrast which, save for the transfer function of the electron microscope, is directly proportional to the wave function at the exit plane of the object.
A second method of image reconstruction is known from the article "Electron image plane off-axis holography of atomic structures" by Hannes Lichte in Advances in Optical and Electron Microscopy, vol. 12, 1991, pp. 25-91. This article describes a linear image reconstruction method using an electron hologram of the object portion to be reconstructed. An electron hologram is formed by interference of frequency components or beams of an electron wave comprising image information with a reference electron wave which directly originates from the electron source. The reference electron wave is only propagated through vacuum and is not modulated by the object, but it does have a spatial coherence with the electron wave comprising the image information. The interference pattern or electron hologram thus recorded is composed in the Fourier space, i.e. the spatial-frequency space or region, of a central frequency domain I.sub.hol,0 (G) and two sidebands I.sub.hol,+/- (G) which are spatially separated from each other. This means that in this method the linear image information is directly separated from the non-linear image information. In fact, the sidebands only contain linear image information, in other words, information about the interferences between frequency components of the electron wave with image information and the undisturbed reference electron wave without image information. However, the central frequency domain is a combination of information about linear as well as non-linear interactions. Linear interactions in the central frequency domain are understood to mean interactions which take place between the on-going beam of the frequency G=0 whose electrons are not scattered in the object, and one of the beams of the frequency G.noteq.0 scattered in the object. Non-linear interactions are understood to mean interactions which take place between two beams of the frequencies G.sub.1 .noteq.0 and G.sub.2 .noteq.0 scattered in the object.
However, a drawback of this method is that the information limit of the high-resolution information which can be obtained from the sidebands of the hologram is limited by the temporal coherence of the source. Temporal coherence is related to the chromatic aberration of the objective lens, to instabilities in the high voltage and the lens currents and to thermal spread of energy of the electrons, which results in an effective focus fluctuation during recording. The information limit is herein understood to mean the smallest object detail which can be retrieved from the electron wave function when only linear interactions are taken into account, in other words, when considering only interferences between a beam with image information and the reference electron wave in electron holography, or between a beam with image information and the on-going beam in conventional high-resolution electron microscopy. This information limit is determined by the temporal coherence of the electron microscope.
Electron holography requires a high spatial coherence so as to realise an interference pattern which is sufficiently rich in contrast between the object wave and the reference wave. Such a high spatial coherence can be achieved, inter alia, by making use of a field emission gun (FEG).
It is known per se, inter alia from Philips Electron Optics Bulletin 130, pp. 53-62 (1991) by P. M. Mul, B. J. H. Bormans and M. T. Otten that the use of a field emission gun (FEG) may lead to a considerable improvement of the resolution due to the higher brightness and the better spread of energy in comparison with a conventional thermionic electron source such as, for example an LaB.sub.6 filament. The supplementary high-resolution information which can be achieved with a FEG is, however, considerably influenced by the afore-mentioned electron-optical aberrations: spherical aberration and defocus, so that a direct interpretation of the experimental observations is impeded. The previously mentioned reconstruction methods, notably focus variation and standard electron holography do not provide a solution to this problem.